This invention relates to a digital-to-analog converter comprising a sigma-delta modulator for generating a 1-bit digital signal modulated synchronously with a clock signal in response to a digital input signal, and output means for supplying an output signal in response to the synchronously modulated digital signal.
Such a DAC is known from, inter alia, an article by Peter J. A. Naus et al., "A CMOS Stereo 16-bit D/A Converter for Digital Audio", IEEE Journal of Solid-State Circuits, Vol. SC-22, No. 3, June 1987, pp. 390-395. The D/A conversion method described in said article is known by the name of "Bit-Stream" conversion. A 16-bit pulse-code modulated digital audio signal of comparatively low clock frequency is oversampled by means of digital interpolation filters until a 17-bit digital audio signal of 256 times the clock frequency is obtained. This 17-bit signal is applied to a second-order sigma-delta modulator (SDM), which on account of its properties is often referred to as a noise shaper. The SDM converts the 17-bit digital signal into a 1-bit digital signal. The advantage of this method is that for the actual digital-to-analog conversion only comparatively simple analog filter means are required, which in the cited article comprise a 1-bit DAC and a first-order low-pass filter. Such a conversion method is not only attractive in the case of audio signals but in principle offers just as many advantages in the case of video signals. However, the clock frequency of an oversampled digital video signal is very high and can be a few hundreds of MHz. As a result of this, the interface between the 1-bit digital signal from the SDM and the analog filter means will pose a difficult technical problem. This interface is needed if the digital and the analog signal processing are effected in different integrated circuits, for example, to preclude interaction or because the digital and the analog pans are manufactured in different technologies.